Vehicles having an electric or hybrid drive need rechargeable batteries (traction batteries), which generally have a modular structure, to operate their electrical drive machine. In many applications, such rechargeable batteries are differently also referred to as storage batteries. In order to now supply the electrical drive machine of the electric or hybrid drive with electrical energy from the batteries, a circuit arrangement is interposed between the battery modules and the drive machine.
The rechargeable batteries, usually based on lithium, used in electrically driven vehicles have only a limited service life on account of parasitic chemical processes in their interior. Their capacity is reduced with each charging/discharging cycle until the individual battery cells or the battery modules consisting of such cells have to be replaced owing to a lack of performance and capacity. Therefore, it is important to accurately observe the aging process of the battery cells or battery modules. Various methods and apparatuses for monitoring the aging state are known from the prior art.
The scientific article “Smith, A. J. et al., J. Electrochem. Soc. 157, A196 (2010)” describes a method which can be used to infer changes in the aging state (change in the SOH: State of Health) of lithium ion battery cells from the so-called Coulombic efficiency. However, a corresponding additional power electronic measuring and regulating device is needed to carry out such a method.
The method according to the disclosure provides the advantage that no additional power electronics are required.
In the method according to the disclosure for determining the Coulombic efficiency CE of battery modules of a rechargeable battery, provision is made for the Coulombic efficiency to be determined by means of a circuit arrangement which is connected to the battery modules and has a plurality of switching modules for selectively connecting each individual battery module of the battery modules in a common current path or for alternatively removing each individual battery module of the battery modules from this current path and at least one power semiconductor element which can be operated in the linear mode and is intended to regulate the current flowing through the current path. In this case, (i) at least one of the battery modules is selected and is connected in the current path by means of the switching modules, while all other battery modules are removed from the current path by means of the switching modules, and (ii) the selected battery module is subjected to at least one discharging process and at least one charging process via the current path, the corresponding current being accurately set during charging and discharging of this battery module in the current path by means of the power semiconductor element which is operated in the linear mode, and the corresponding charge quantities Qab, Qzu during charging and discharging or variables proportional to these charge quantities being determined by integrating the current over time. The Coulombic efficiency CE defined as
      C    E    =            Q      ab              Q      zu      can then be determined from the charge quantities Qab, Qzu or variables proportional to the latter. In the simplest case, each of the battery modules consists of an individual battery cell. Alternatively, each of the battery modules consists of a series circuit of a plurality of battery cells.
The circuit arrangement is interposed between the battery modules of the rechargeable battery and a consumer to be supplied by the battery or batteries, each battery module being connected to a switching module of the circuit arrangement. During normal operation, the switching modules are used to select individual battery modules for this voltage supply and to connect them to one another in a current path. Such a circuit arrangement is known as a battery direct converter. The battery direct converter can be interposed directly, that is to say without further intermediate elements, between the battery modules, on the one hand, and the electrical consumer to be supplied by the battery modules.
The essence of the disclosure is to control a power semiconductor element in the current path of the circuit arrangement in such a manner that said element is at least sometimes in the linear mode and the current through the battery cells of the corresponding battery module is regulated very accurately with the aid of this linearly operated power semiconductor element in accordance with current regulation in the charger. A power semiconductor element which can be operated in this manner is generally present in battery direct converters anyway. Therefore, the very accurate setting of the charging or discharging current, which is needed to determine the Coulombic efficiency CE, can be easily implemented without additional power electronics. Only the control of said power semiconductor element would have to be supplemented in order to carry out the method according to the disclosure. However, such control can manage without power electronic components.
The consumer to be supplied by the battery modules is preferably a multiphase electrical consumer, in particular a multiphase electrical machine. In this case, the battery direct converter is a multiphase direct converter which can be interposed directly between the battery modules of the batteries, on the one hand, and the multiphase electrical consumer to be supplied by the battery modules. In this case, the battery modules can be connected in a number of current paths corresponding to the number of phases.
According to one advantageous development of the disclosure, one of the power semiconductor elements of the switching modules forms the power semiconductor element for regulating the current flowing through the current path. In this embodiment, the power semiconductor elements of the switching modules are controlled by means of a control device and are operated in the linear mode in order to set the electrical current during the charging process and the discharging process.
Each of the switching modules advantageously has two power semiconductor elements which act as semiconductor valves and two freewheeling diodes. They are connected in a bridge circuit arrangement in the form of a half-bridge. In this case, one of the two semiconductor valves is connected in parallel with one of the two freewheeling diodes. The two parallel circuits with the one semiconductor valve and the one freewheeling diode each are connected in a series circuit, thus producing the half-bridge. This series circuit is connected to the corresponding battery module. Such switching modules are known from direct converters, for example, and are used there for so-called “cell balancing”, the equalization of the state of charge between the individual battery cells or battery modules. For this purpose, the battery cells or battery modules are preferably connected, by means of the switching modules, in the current path whose state of charge is relatively high.
According to another advantageous development of the disclosure, the circuit arrangement also has an inverter for connection to an electrical consumer which requires AC voltage or alternating current.
Provision is advantageously made for a power semiconductor element of the inverter to form the power semiconductor element for regulating the current flowing through the current path. In this embodiment, this power semiconductor element of the inverter is therefore controlled by means of a control device and is operated in the linear mode in order to set the electrical current during the charging process and the discharging process.
According to yet another advantageous development of the disclosure, the inverter has a DC voltage intermediate circuit. An intermediate circuit capacitor is connected in this DC voltage intermediate circuit.
The selected battery module is preferably discharged via                the electrical consumer connected to the inverter, or        a connectable load resistor of the circuit arrangement, or        a short circuit in the inverter. The connectable load resistor is advantageously arranged in the intermediate circuit and is selectively connected or disconnected by means of a controllable switching device (a contactor).        
The selected battery module is preferably charged via a charger connected to the current path.
The disclosure also relates to a circuit arrangement for determining the Coulombic efficiency of battery modules of a rechargeable battery. The circuit arrangement comprises a plurality of switching modules for selectively connecting each individual battery module of the battery modules in a common current path or for alternatively removing each individual battery module of the battery modules from the current path and at least one power semiconductor element which can be operated in the linear mode and is intended to regulate the current flowing through the current path. The switching modules are set up to select at least one of the battery modules and to connect it in the current path and to remove all other modules from the current path. The circuit arrangement is set up to subject the selected battery module to at least one discharging process and at least one charging process via the current path, the corresponding current in the current path being able to be accurately set during charging and discharging of this battery module by means of the power semiconductor element which is operated in the linear mode, and the circuit arrangement having means for determining the corresponding charge quantities by integrating the current over time during the charging process and the discharging process. The circuit arrangement also comprises a control device for controlling the power semiconductor element in the linear mode in order to set the electrical current during the charging process and the discharging process.